“If they are not dependent on foreign suppliers,” he said, then “even the most targeted sanctions on, and monitoring of” countries that might assist North Korea, “will be mostly futile.”

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A photo from North Korea’s state news agency in 2010 purporting to show Kim Jong-il, the country’s leader at the time, at the February 8 Vinalon Plant in Hamhung. The plant is suspected of making missile fuel.Credit
Korean Central News Agency, via Reuters

Short of war or the country’s collapse, he added, “There’s nothing to stop this program from becoming a monster.”

The finding is based on satellite imagery, a technical analysis of UDMH production methods, information from a North Korean official who defected, and a set of obscure North Korean technical documents.

Jeffrey Lewis, who directs the Middlebury center’s East Asia program, had been hunting for weeks for hints of UDMH production.

“There are no real, obvious signatures for it,” he said, because it can be made with common chemicals like chlorine and ammonia using a variation of a process developed in 1906. India, while quietly developing its missile program in the 1970s, had produced UDMH in an old sugar factory.

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The breakthrough came when his team found and translated a set of highly technical articles in an official North Korean science journal, Chemistry and Chemical Engineering, that referred to the fuel.

The articles, which ran between 2013 and 2016, discussed mundane matters like managing highly toxic wastewater, a notorious problem in UDMH production. But they betrayed suspiciously sophisticated knowledge. One explored methods for improving purity, crucial in advanced missiles.

“They don’t read like this is a speculative or nascent endeavor,” Mr. Lewis said. “They read like this is a problem they’ve been working on for a while,” describing problems a country would encounter only after producing large quantities of the fuel.

But the documents betrayed something else: conspicuous secrecy. Unlike others in the journal, these three articles listed no affiliation or biography for its authors, suggesting their work was more sensitive than it looked.

Mr. Lewis’s team cross-referenced the authors’ names with every scrap of North Korean chemical research they could find, until they noticed something odd. One of the authors, Cha Seok Bong, had published three papers on more anodyne matters out of a place called the February 8 Vinalon Plant, based in remote Hamhung.

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Another state news agency photo was said to show the launch of a Hwasong-14 intercontinental ballistic missile in July. The fuel UDMH makes such long-range weapons possible.Credit
Korean Central News Agency, via Associated Press

It was an odd location for a highly trained rocket fuel specialist to work. The plant normally produces vinalon, a cheap synthetic material sometimes called “juche” fiber — a reference to North Korea’s tenet of juche, or self-reliance — that is often used in North Korean textiles and uniforms.

But it had long been, Mr. Lewis said, “our No. 1 candidate for UDMH production.”

His team had initially flagged the plant after scrutinizing, in painstaking detail, satellite images of Hamhung for clues.

The remote city is not an obvious home for sensitive military sites. Sitting on the country’s eastern coast, it is exposed to airstrikes, like the American bombing missions that devastated it in the Korean War.

But Ko Chong-song, a North Korea official who defected in the early 1990s, indicated in a 2001 book that it was the center of secret military chemical work. The Central Intelligence Agency had suspected as much since at least 1969, when it published a secret assessment of chemical production in Hamhung.

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Now, Mr. Lewis’s team, looking again at the plant, noticed two unusually large wastewater pools, which aligned with standard UDMH production methods — and with the paper describing wastewater challenges. And Kim Jong-un, the North Korean leader, they found, had made a series of trips to the plant, underscoring its importance.

“That’s about as clear-cut as it gets,” Mr. Lewis said.

While the finding reveals important information about the extent of North Korea’s progress, it may come too late for the United States to act.

The country has most likely already stockpiled enough fuel to fight an extended war, Mr. Lewis said. And the fuel is designed to remain potent for years. Soviet UDMH lasted so long that, after the country collapsed, the United States had to help de-fuel its ICBMs.

Asked how North Korea could have so extensively developed this fuel without apparent outside notice, Mr. Lewis said outside analysts too often saw the country as primitive and backward.

“If you watch them in satellite photos and read their technical publications, it looks like a totally different country,” he said.

He added, sighing, “We’re in full-scale denial about North Korea’s capabilities.”